Abstract/Summary

The impact of selected observing systems on forecast skill is explored
using the European Centre for Medium-Range Weather Forecasts (ECMWF)
40-yr reanalysis (ERA-40) system. Analyses have been produced for
a surface-based observing system typical of the period prior to 1945/1950,
a terrestrial-based observing system typical of the period 1950-1979
and a satellite-based observing system consisting of surface pressure
and satellite observations. Global prediction experiments have been
undertaken using these analyses as initial states, and which are
available every 6 h, for the boreal winters of 1990/1991 and 2000/2001
and the summer of 2000, using a more recent version of the ECMWF
model. The results show that for 500-hPa geopotential height, as
a representative field, the terrestrial system in the Northern Hemisphere
extratropics is only slightly inferior to the control system, which
makes use of all observations for the analysis, and is also more
accurate than the satellite system. There are indications that the
skill of the terrestrial system worsens slightly and the satellite
system improves somewhat between 1990/1991 and 2000/2001. The forecast
skill in the Southern Hemisphere is dominated by the satellite information
and this dominance is larger in the latter period. The overall skill
is only slightly worse than that of the Northern Hemisphere. In the
tropics (20 degrees S-20 degrees N), using the wind at 850 and 250
hPa as representative fields, the information content in the terrestrial
and satellite systems is almost equal and complementary. The surface-based
system has very limited skill restricted to the lower troposphere
of the Northern Hemisphere. Predictability calculations show a potential
for a further increase in predictive skill of 1-2 d in the extratropics
of both hemispheres, but a potential for a major improvement of many
days in the tropics. As well as the Eulerian perspective of predictability,
the storm tracks have been calculated from all experiments and validated
for the extratropics to provide a Lagrangian perspective.